This application claims the priority of German Patent Application No. 198 04 286.8, filed Feb. 4, 1998.
The present invention relates to a reactor for the catalytic chemical reaction of a gaseous or liquid starting product to an end product that contains a gaseous or liquid constituent which can be selectively separated by means of membrane diffusion.
Reactors which are suitable for the catalytic chemical reaction of a gaseous or liquid reaction starting product to a reaction end product, wherein the reaction end product contains a gaseous or liquid reaction constituent that can be selectively separated by means of membrane diffusion, are in use, for example, for the water vapor reforming of methanol. Such reactors have a reaction space into which the reaction starting product can flow and which contains a suitable catalyst material. In the water vapor reforming of methanol, a hydrogen-rich mixture containing a certain part of carbon monoxide is obtained as the reaction end product. It is known to use such methanol reforming reactors in mobile systems in fuel-cell-operated motor vehicles in order to obtain the hydrogen required for the fuel cells from liquid methanol. Since carbon monoxide has a damaging influence on the fuel cells, the reaction end product of the methanol reforming reaction requires a corresponding treatment. The selective separation of the hydrogen by means of a membrane diffusion is known as one possibility. Particularly for a mobile use in automotive engineering, it is desirable for reasons of dynamics and space that the reactor have a construction which is as compact and as light as possible and has few components. In this case, as few control units and automatic control units as possible should be used, while a high efficiency is simultaneously endeavored.
U.S. Pat. No. 4,981,676 discloses a reactor which is used for the water vapor reforming of a hydrocarbon, particularly methane, and contains a cylindrical-ring-shaped reaction space that is filled with a suitable catalyst pellet charge. To the outside, the reaction space is bounded by a metallic wall which can be heated from the outside, while the inner boundary of the reaction space is formed by a tube that acts as a hydrogen separating membrane and has a correspondingly porous ceramic tube wall. The diameter of the membrane tube is typically of the order of 25 mm and greater.
European Patent Document EP 0 228 885 B1 discloses a reactor for the decomposition of hydrogen sulfide for the purpose of obtaining hydrogen, which is constructed as a so-called tube bundle reactor. The reactor contains a bundle of spaced reaction tubes which form the reaction space. The reaction tubes are implemented by means of a porous wall such that they act as a selectively hydrogen-separating membrane as well as contain a suitable catalyst material. The wall thickness of the tube is approximately 1 mm and the tube diameter is a multiple thereof.
As a special membrane for the selective separation of certain constituents from a substance mixture by means of membrane diffusion, so-called hollow fibers are known, as described, for example, in European Published Patent Application EP 0 186 129 A2 and U.S. Pat. Nos. 4,175,153, 5,474,680, and 5,558,936. These hollow fibers have a porous wall made, for example, of ceramic or glass material, through which one selected constituent of a substance mixture can move. Such hollow fibers are suggested, for example, for the selective separation of hydrogen from a hydrogen-containing mixture. The hollow fibers typically have a diameter of the order of several hundredths of a millimeter to a few millimeters and wall thicknesses of the order of 1 xcexcm to a few 100 xcexcm. U.S. Pat. No. 4,175,153 specifically discloses a hydrogen separating stage with a cylindrical housing in which a bundle of such hollow fibers is placed in a mutually spaced manner. The hydrogen-containing mixture, such as the reaction end product of a water vapor reforming of methanol, is introduced into the housing on the face side and is guided through the interior of the hollow fibers arranged in parallel. The hydrogen is selectively diffused through the hollow fiber walls and, as the result, is separated from the remaining mixture constituents. U.S. Pat. No. 4,175,153 discloses, as a further application possibility, the use of such hydrogen-permeable hollow fibers as a fuel cell electrode in a fuel cell or in catalyst-filled hydrogen-generating reactors. A hydrogen generating reactor with several small thin hollow diffusion tubes which traverse a catalyst bed is disclosed in Austrian Patent Document AT 247 284.
One object of the present invention is to provide a reactor that can be constructed in a relatively compact and easy manner.
Another object of the invention is to provide a reactor that permits a high reaction efficiency.
Still another object of the invention is to provide such a reactor which, during a cold start, permits a fast heating of its membrane-type hollow fibers to an operating temperature suitable for the effective reaction constituent separation.
These and other objects and advantages are achieved by the reactor according to the invention, in which porous hollow fibers are placed directly in the reaction space and operate there as membranes for the selective separation of a selectively separable constituent contained in the reaction end product from the residual reaction end product. In this case, the constituent of the reaction end product formed in the reaction space which is to be separated, diffuses from the reaction space through the porous hollow fiber wall into the inner duct of the respective hollow fiber, and can be withdrawn from the reaction space by way of the hollow fiber ducts. The separating membranes are therefore situated directly at the site at which the catalytic chemical reaction takes place and permit a separation of the concerned constituent, such as hydrogen, from a hydrogen-containing mixture, with high selectivity and yield. By the separation of this constituent while at the site of the chemical reaction, the reaction equilibrium can be displaced to the side of the desired constituent.
The positioning of the membrane-type hollow fibers in the reaction space itself benefits a compact reactor construction, and by the use of membrane-type hollow fibers, in the case of a given constructional volume, a comparatively high membrane surface area can be made available for the diffusive separation of the desired constituent. The compact reactor construction with hollow-fiber separating membranes integrated in the reaction space has a favorable effect on achieving fast reaction times under fluctuating load conditions. This makes the reactor particularly suitable for motor vehicles, where transient operating phases occur frequently. In particular, the reactor can be used as a methanol reforming reactor for obtaining hydrogen by the water vapor reforming of methanol in fuel-cell-operated motor vehicles.
The porous hollow fibers are constructed of a ceramic or glass material and are provided with a selectivity-increasing metallic coating. The metal material of this coating is selected such that the selectivity of the separation of the desired reaction constituent is increased. Furthermore, devices are provided for heating the porous hollow fibers by applying an electric voltage to their metallic coating. This arrangement heats the hollow fibers in a simple manner, so that, for example, during a starting phase, they can very rapidly be brought to a raised temperature required for an effective separating function.
In the case of a reactor according to another embodiment of the present invention, the catalyst material is present in a catalyst layering which is formed by (1) a catalyst pellet charge; (2) a porous carrier body, into which the catalyst material is charged; or (3) a metallic foam catalyst body that consists of the mostly metallic catalyst material. The porous hollow fibers extend in a parallel arrangement with a predetermined mutual transverse spacing through the catalyst layering.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.